Here at PJC Reptiles we stick mainly to the rule of Threes: Three years old, three feet long, 300 grams. But there are always exceptions to some rules.
Example:
2005 1.0 Amel Motley/Stripe het caramel, this guy will never be over 3ft and maybe, just maybe someday, he will be over 300 grams. He is perfect in size with his muscle tone and head structure for his size, parasite free, has never refused a meal, all around good example of a healthy corn. This is his first year breeding and hoping all goes well with his first breeding mate. I suppose only time will tell how being a small male effects reproduction and offspring. But than again, this is a male. Would I have bred him if he was a she? Probably not.
I am sure there are other breeders out there that will have other guide lines and it works for them. This is just what seems to work best for us.